A subset of patients presenting with hypereosinophilic syndrome (HES) have clinical features consistent with a myeloproliferative disorder. These patients have aggressive disease characterized by tissue fibrosis, including endomyocardial fibrosis and myelofibrosis and a poor prognosis. The peripheral blood mononuclear cells of these patients have an interstitial deletion in chromosome 4, leading to the formation of the imatinib-sensitive FIP1L1/PDGFRA fusion gene, thus fulfilling the WHO criteria for a diagnosis of chronic eosinophilic leukemia (CEL). We have established a novel RT-PCR assay to test for FIP1L1/PDGFRA fusion gene in peripheral blood samples of patents with eosinophilia. Positive patients are followed over time and treatment responses are monitored using this novel RT-PCR assay. However, the majority of patients with eosinophilia do not have FIP1L1/PDGFRA fusion gene and are classified as idiopathic HES. Very little is known about surface and soluble IL-5 receptor alpha expression in patients with eosinophilia. IL5 receptor alpha (IL-5Ra) is a surface receptor found on eosinophils, mast cells, basophils and their precursors. Although it has been clearly demonstrated that IL-5, IL-3 and GM-CSF down regulate surface expression of IL-5 receptor alpha (IL-5Ra) on normal eosinophils with a concomitant increase in production of the soluble form (sIL-5Ra), little is known about IL-5Ra regulation in patients with increased eosinophils (HES patients) or increased mast cells (mastocytosis patients). Surface expression of IL-5Ra on eosinophils and mast cells was assessed by flow cytometry. Soluble IL-5Ra levels were measured by capture ELISA in serum from patients with eosinophilia (n=11), D816KIT positive systemic mastocytosis (n=8) and normal volunteers (n=7). Results revealed that IL5-Ra expression had an inverse correlation with eosinophil levels in patients with eosinophilia (r=-0.85, p=0.0037), but not in normal controls. In contrast, serum sIL-5Ra was detectable in all 11 patients with eosinophilia, but in only 2 of 7 normal volunteers. Interestingly, serum sIL5Ra levels were comparable in the patients with eosinophilia (GM 957, range 214-15,450 ng/ml) and systemic mastocytosis (GM 847, range 355-1,805 ng/ml) despite eosinophil counts within the normal range in 4/8 mastocytosis patients. Basophil counts were also comparable between the two groups (GM 55 vs.32/mm3, respectively). These data are consistent with an in vivo IL-5R regulatory pathway in human eosinophils and mast cells similar to that described in vitro and involving a balance between surface and soluble receptor levels. This may have important implications in the face of recently developed monoclonal antibody therapies against IL-5 and its receptor.